Irradiation of 2-azidofluorene in an aqueous solution containing 2‘-deoxyguanosine (dG) gives good yields of 8-(2-fluorenylamino)dG. This is the C8 adduct implicated in the carcinogenicity of 2-aminofluorene, and formed in vivo from the reaction of DNA-guanine and an ester of N-hydroxy-2-aminofluorene. Flash photolysis of the azide reveals two intermediates on the pathway that forms this adduct, the 2-fluorenylnitrenium ion, and a subsequent longer-lived species formed in the reaction of this ion with dG. A number of pieces of evidence identify this later intermediate as the initial C8 adduct derived from addition of the nitrenium ion to the C8 carbon prior to loss of the C8 proton. Both spectroscopic and kinetic analyses show that the latter actually exists in both a cationic acid form and a base form, with a pK a for the acid of 3.9. The base form is a tautomer of the final product and is the species present at pH 7. There is also evidence that the reaction of the nitrenium ion and dG that forms this intermediate proceeds directly by addition at C8. In other words, the substitution of ArNH+ for H+ at the C8 position of 2‘-deoxyguanosine is a straightforward electrophilic aromatic substitution.

In order to assess the extent of DNA sequence variation in cattle, introns and exons from both the leptin and Amyloid Precursor Protein (APP) genes have been sequenced in a panel of DNAs derived from 22 diverse animals. Direct DNA sequencing of PCR products was used; thus, 44 chromosomes were studied. Polymorphisms were identified by manual scanning of sequence chromatograms and computerized sequence analysis. Twenty Single Nucleotide Polymorphisms (SNPs) were detected in 1788 bp sequenced from the leptin gene, giving a frequency of 1 SNP per 89 bp. Twenty-four SNPs were detected in a 458-bp fragment of the APP gene; 23 of the polymorphisms were contained in a 302-bp intron 16 fragment. This equates to an SNP frequency of 1 per 13 bp for the intron. We can thus conclude that this portion of the bovine APP gene constitutes a hypermutable region. Nucleotide sequence diversity values of 0.019 and 0.0026 were obtained for APP and leptin respectively.

The 2-carbazolylnitrenium ion and its 9-methyl analog have been investigated by laser flash photolysis. These electrophiles were generated by irradiation of 2-azido-9H-carbazole and 9-methyl-2-azido-9H-carbazole in solutions of 20% acetonitrile in water. Kinetic and spectral evidence is presented for three forms of the 2-carbazolylnitrenium ion, the monocation, its dication conjugate acid and a neutral quinonoid-like conjugate base obtained by loss of the 9-NH proton. The 9-methyl system, which cannot form the neutral species, exists as dication and monocation. Acidity constants relating the various forms and rate constants for their reaction with water, bromide ion and azide ion are obtained by kinetic analyses.

4-Methoxybenzyl cations bearing α-(N,N-dimethylcarbamoyl) and α-(N,N-dimethylthiocarbamoyl) substituents have been generated photochemically upon irradiation of precursors with pentafluorobenzoate or 4-methoxybenzoate leaving groups. The ions have been observed with flash photolysis in 40:60 acetonitrile:water and in 50:50 methanol:water, and rate constants were measured for their decay in solvent alone and for their capture by azide ion. The cations so studied and their lifetimes in 40% acetonitrile are 6, ArC + H-CONMe 2 , 0.6 µs; 2, ArC + H-CSNMe 2 , 7 ms; and 4, ArC + (CH 3 )-CSMe 2, 6 ms, where Ar = 4-MeOC 6 H 4 . The cation 4 reacts with solvent by elimination of a proton from the α-methyl group, and the rate constant for solvent addition must be less than 1 s -1 . The CSNMe 2 substituted cations are 10 5 -10 7 -fold longer lived than analogs where the thioamide group has been replaced with an αmethyl. The UV-visible absorption spectra of these two cations also show significant differences from those of typical 4-methoxybenzyl cations. Thus, both the lifetimes and spectra point to a strong interaction of the benzylic centre with the thioamide group.

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